A lithographic printing plate precursor

ABSTRACT

A lithographic printing plate precursor is disclosed including a support and a coating comprising (i) a photopolymerisable layer including a polymerisable compound, a borate compound and/or a photoinitiator and (ii) a toplayer provided above the photopolymerisable layer; characterized in that the toplayer has a thickness comprised between 0.1 g/m2 and 1.75 g/m2 and includes an infrared absorbing compound which includes a thermocleavable group which transforms into a group which is a stronger electron-donor upon exposure to heat and/or IR radiation, and is capable of forming a print-out image upon exposure to heat and/or IR radiation.

TECHNICAL FIELD

The invention relates to a novel lithographic printing plate precursor.

BACKGROUND ART

Lithographic printing typically involves the use of a so-called printingmaster such as a printing plate which is mounted on a cylinder of arotary printing press. The master carries a lithographic image on itssurface and a print is obtained by applying ink to said image and thentransferring the ink from the master onto a receiver material, which istypically paper. In conventional lithographic printing, ink as well asan aqueous fountain solution (also called dampening liquid) are suppliedto the lithographic image which consists of oleophilic (or hydrophobic,i.e. ink-accepting, water-repelling) areas as well as hydrophilic (oroleophobic, i.e. water-accepting, ink-repelling) areas. In so-calleddriographic printing, the lithographic image consists of ink-acceptingand ink-abhesive (ink-repelling) areas and during driographic printing,only ink is supplied to the master.

Lithographic printing masters are generally obtained by the image-wiseexposure and processing of a radiation sensitive layer on a lithographicsupport. Imaging and processing renders the so-called lithographicprinting plate precursor into a printing plate or master. Image-wiseexposure of the radiation sensitive coating to heat or light, typicallyby means of a digitally modulated exposure device such as a laser,triggers a physical and/or chemical process, such as ablation,polymerization, insolubilization by cross-linking of a polymer or byparticle coagulation of a thermoplastic polymer latex, solubilization bythe destruction of intermolecular interactions or by increasing thepenetrability of a development barrier layer. Although some plateprecursors are capable of producing a lithographic image immediatelyafter exposure, the most popular lithographic plate precursors requirewet processing since the exposure produces a difference in solubility ordifference in rate of dissolution in a developer between the exposed andthe non-exposed areas of the coating. In positive working lithographicplate precursors, the exposed areas of the coating dissolve in thedeveloper while the non-exposed areas remain resistant to the developer.In negative working lithographic plate precursors, the non-exposed areasof the coating dissolve in the developer while the exposed areas remainresistant to the developer. Most lithographic plate precursors contain ahydrophobic coating on a hydrophilic support, so that the areas whichremain resistant to the developer define the ink-accepting, henceprinting areas of the plate while the hydrophilic support is revealed bythe dissolution of the coating in the developer at the non-printingareas.

Photopolymer printing plates rely on a working-mechanism whereby thecoating—which typically includes free radically polymerisablecompounds—hardens upon exposure. “Hardens” means that the coatingbecomes insoluble or non-dispersible in the developing solution and maybe achieved through polymerization and/or crosslinking of thephotosensitive coating upon exposure to light and/or heat. Photopolymerplate precursors can be sensitized to blue, green or red light i.e.wavelengths ranging between 450 and 750 nm, to violet light i.e.wavelengths ranging between 300 and 450 nm or to infrared light i.e.wavelengths ranging between 750 and 1500 nm. Optionally, the exposurestep is followed by a heating step to enhance or to speed-up thepolymerization and/or crosslinking reaction.

In general, a toplayer or protective overcoat layer over the imageablelayer is required to act as an oxygen barrier to provide the desiredsensitivity to the plate. A toplayer typically includes water-soluble orwater-swellable polymers such as for example polyvinylalcohol. Besidesacting as barrier for oxygen, the toplayer should best be easilyremovable during processing and be sufficiently transparent for actinicradiation, e.g. from 300 to 450 nm or from 450 to 750 nm or from 750 to1500 nm.

The classical workflow of photopolymer plates involves first an exposurestep of the photopolymer printing plate precursor in a violet orinfrared platesetter, followed by an optional pre-heat step, a wash stepof the protective overcoat layer, an alkaline developing step, and arinse and gum step. However, there is a clear evolution in the directionof a simplified workflow where the pre-heat step and/or wash step areeliminated and where the processing and gumming step are carried out inone single step or where processing is carried out with a neutral gumand then gummed in a second step. Alternatively, on-press processingwherein the plate is mounted on the press and the coating layer isdeveloped by interaction with the fountain and/or ink that are suppliedto the plate during the press run, has become very popular. During thefirst runs of the press, the non-image areas are removed from thesupport and thereby define the non-printing areas of the plate.

In order to be able to evaluate the lithographic printing plates forimage quality, such as for example image resolution and detail rendering(usually measured with an optical densitometer) before mounting them onthe press, the lithographic printing plate precursors often contain acolorant such as a dye or a pigment in the coating. Such colorantsprovide, after processing, a contrast between the image areas containingthe colorant and the hydrophilic support where the coating has beenremoved which enables the end-user to evaluate the image quality and/orto establish whether or not the precursor has been exposed to light.Furthermore, besides allowing for the evaluation of the image quality, ahigh contrast between the image and the hydrophilic support is requiredin order to obtain a good image registration (alignment) of thedifferent printing plates in multi-colour printing in order to ensureimage sharpness (resolution) and a correct rendering of the colours inthe images present.

However, for photopolymer lithographic printing plates which areprocessed on-press and thus development of the plate is not carried outbefore mounting the plate on the press, a previous inspection anddiscrimination of the plate including colorants is not possible. Asolution has been provided in the art by including components to thecoating which are able to form upon exposure a so-called “print-outimage”, i.e. an image which is visible before processing. In thesematerials however, often the photo-initiating system is a reactingcomponent, which induces formation of the print-out image upon exposure,and therefore the lithographic differentiation may be reduced.

Formation of a print-out image for violet sensitized photopolymersystems have been disclosed in for example U.S. Pat. Nos. 3,359,109;3,042,515; 4,258,123; 4,139,390; 5,141,839; 5,141,842; 4,232,106;4,425,424; 5,030,548; 4,598,036; EP 434 968; WO 96/35143 and US2003/68575.

The formation of a print-out image is also known for heat-sensitivephotopolymer lithographic printing plates. Such plates are usuallyimage-wise exposed by an IR laser and often comprise, beside an IR dyeas a light-to-heat conversion compound, also a dye which absorbs in thevisible light wavelength range and changes colour upon heating. Thiscolour change can be obtained for example with a heat-decomposable dyewhich bleaches upon heating such as disclosed in EP 897 134, EP 925 916,WO 96/35143, EP 1 300 241. Alternatively, this heat-induced colourchange can be the result of a shift of the absorption maximum of avisible dye as disclosed in EP 1 502 736 and EP 419 095. A problemassociated with these prior art materials where the print-out image isformed by a heat-induced reduction of the visible light absorption or bya switch from a highly colored to a weakly colored coating, is that theobtained print-out images are characterized by only a low contrastbetween the exposed and the non-exposed areas, high levels of dyes arerequired, and/or an increased risk of contamination of thedevelopment/rinse section.

Contrast-providing colorants obtained from the so-called leuco dyes thatswitch colour upon changes in pH, temperature, UV etc, have been widelyused in the art. The leuco dye technology involves a switch between twochemical forms whereby one is colourless. If the colour switch is causedby for example pH or temperature, the transformation is reversible.Irreversible switches are typically based on redox reactions.

The use of contrast-providing colorants obtained from leuco dyes thatbecome coloured in the presence of a thermal acid generator, isdescribed for example, in U.S. Pat. Nos. 7,402,374; 7,425,406 and7,462,440. The colouring of the printing areas is initiated byimage-wise exposure whereby the image areas are visualized beforeperforming development of the plate precursor. However, only a weakimage contrast which fades away in time is obtained with this leuco dyetechnology and, moreover, high exposure energies are required togenerate a contrast.

EP 2 297 611 discloses an imaging element comprising a topcoat layerdisposed on a photopolymerisable imageable layer comprising awater-soluble polymer binder and a composition that is capable ofchanging colour upon exposure to infrared radiation which comprises anacid-generating compound, an infrared radiation absorbing compound andoptionally one or more compounds that generate a colour in the presenceof the acid.

Thermochromic dye technology involves the design of an IR dye containinga thermocleavable group whereby a colour shift is obtained upon exposurewith heat and/or light. This technology offers lithographic contrastwhich is enhanced by increasing either the thermochromic dyeconcentration or the exposure energy. However, this technology isespecially suitable for thermofuse plates—i.e. plates including animage-recording layer that works by heat-induced particle coalescence ofa thermoplastic polymer latex,—and does not work well in thephotosensitive layer of photopolymer based printing plates. Indeed, onlyan acceptable contrast in such printing plates is feasible when exposedby very high laser energy and/or when a substantially high concentrationof the thermochromic dye is incorporated in the coating.

In conclusion, there is still a need for lithographic printing platesbased on photopolymerisation including coating formulations which offeran improved contrast between the image areas and background areas uponimaging and which are preferably designed for direct on-pressdevelopment.

SUMMARY OF INVENTION

It is therefore an object of the present invention to provide anegative-working printing plate based on photopolymerisation whichoffers an excellent visual contrast upon imaging, even beforeprocessing.

This object is realised by the printing plate precursor defined in claim1 with preferred embodiments defined in the dependent claims. Theprinting plate material of the present invention has the specificfeature that it contains a two layer coating of which the toplayerincludes an infrared absorbing compound capable of forming a colouredcompound—whereby a print-out image is formed—upon exposure to IR lightand/or heat. A coloured compound is a compound which is visible for thehuman eye, typically the portion of the electromagnetic spectrum that isvisible to the human eye are wavelengths from about 390 to 700 nm.

According to the current invention, it was surprisingly found that byincorporating the infrared absorbing compound capable of forming acoloured compound in the toplayer, results in a very high visualcontrast. It has been observed that upon heat and/or light exposure ofthe coating according to the present invention, a clear print-out imageis formed even at low exposure energy levels; for example below 150mJ/m².

The CIE 1976 colour distance ΔE measured before development and afterexposure between exposed (image) and non-exposed (non-image) areas—forexample with an energy density between 70 and 190 mJ/m², more preferablybetween 75 and 150 mJ/m², most preferably between 80 and 120 mJ/m²,preferably has a value of at least 2. As a result, efficient platedetection by punch bender and/or registration systems is possible, theneed for an additional ink jet print system to inkjet plate informationafter imaging and/or for a laborous precontrole of the plate by means offor example an acidic gum, is not needed anymore.

The development is preferably carried out by treating the precursor witha gum solution, however more preferably by mounting the precursor on aplate cylinder of a lithographic printing press and rotating the platecylinder while feeding dampening liquid and/or ink to the precursor.

It is a further object of the present invention to provide a method formaking a lithographic printing plate comprising the steps of:

-   -   image-wise exposing the printing plate precursor including the        coating as defined above to heat and/or IR radiation whereby a        lithographic image consisting of image areas and non-image areas        is formed and whereby a colour change in the imaged areas is        induced;    -   developing the exposed precursor.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention. Specificembodiments of the invention are also defined in the dependent claims.

DESCRIPTION OF EMBODIMENTS The Lithographic Printing Plate Precursor

The lithographic printing plate precursor according to the presentinvention is negative-working, i.e. after exposure and development thenon-exposed areas of the coating are removed from the support and definehydrophilic (non-printing) areas, whereas the exposed coating is notremoved from the support and defines oleophilic (printing) areas. Thehydrophilic areas are defined by the support which has a hydrophilicsurface or is provided with a hydrophilic layer. The hydrophobic areasare defined by the coating, hardened upon exposing, optionally followedby a heating step. Areas having hydrophilic properties means areashaving a higher affinity for an aqueous solution than for an oleophilicink; areas having hydrophobic properties means areas having a higheraffinity for an oleophilic ink than for an aqueous solution.

“Hardened” means that the coating becomes insoluble or non-dispersiblefor the developing solution and may be achieved through polymerizationand/or crosslinking of the photosensitive coating, optionally followedby a heating step to enhance or to speed-up the polymerization and/orcrosslinking reaction. In this optional heating step, hereinafter alsoreferred to as “pre-heat”, the plate precursor is heated, preferably ata temperature of about 80° C. to 150° C. and preferably during a dwelltime of about 5 seconds to 1 minute.

The coating contains a toplayer and at least one layer including aphotopolymerisable composition, said layer is also referred to as the“photopolymerisable layer”. The toplayer is provided on top of thephotopolymerisable layer. The coating may further include other layerssuch as for example an intermediate layer, located between the supportand the photopolymerisable layer and/or between the top layer and thephotopolymerisable layer, an adhesion improving layer and/or otherlayers.

The coating of the printing plate precursor is preferably capable ofbeing developed on-press with dampening liquid and/or ink.

The printing plate of the present invention is characterized in that itcan be exposed at a low energy density, i.e. below 190 mJ/m²; preferablybetween 70 and 190 mJ/m²; more preferably between 75 and 150 mJ/m² andmost preferably between 80 and 120 mJ/m².

Toplayer

The coating includes a toplayer or protective overcoat layer whichpreferably acts as an oxygen barrier layer. Low molecular weightsubstances present in the air may deteriorate or even inhibit imageformation and therefore a toplayer is applied to the coating. A toplayershould preferably be easily removable during development, adheresufficiently to the photopolymerisable layer or optional other layers ofthe coating and should preferably not inhibit the transmission of lightduring exposure. The toplayer is provided on top of thephotopolymerisable layer.

The toplayer includes an infrared absorbing compound which is capable offorming a coloured compound—whereby a print-out image is formed—uponexposure to infrared light and/or heat. The infrared absorbing compoundis preferably an infrared absorbing dye, also referred to as IR dye. Thecolour-forming IR dye is also referred to herein as thermochromicinfrared absorbing dye or thermochromic IR dye. The thermochromic IR dyehas a main absorption in the infrared wavelength range of theelectromagnetic spectrum—i.e. a wavelength range between about 750 and1500 nm—and does preferably not have a substantial light absorption inthe visible wavelength range of the electromagnetic spectrum—i.e. awavelength range between 390 and 700 nm. The thermochromic compound,preferably a dye, includes at least one thermocleavable group which istransformed by a chemical reaction, induced by exposure to IR radiationor heat, into a group which is a stronger electron-donor. As a result,the exposed thermochromic dye absorbs substantially more light in thevisible wavelength range of the electromagnetic spectrum, or in otherwords, the thermochromic IR dye undergoes a hypsochromic shift whereby avisible image is formed, also referred to as print-out image. Theformation of this print-out image is clearly different from a process ofthe prior art where a compound changes from an essentially colourlesscompound into a pale-coloured to coloured compound. These compoundstypically change absorption from the UV wavelength range of theelectromagnetic spectrum to the visible wavelength range of theelectromagnetic spectrum, i.e. these compounds typically have abatochromic shift. The contrast of the print-out image obtained by sucha process is much weaker compared to the colour forming processdescribed above of the thermochromic dyes of the present invention.

The contrast of the print-out image may be defined as the differencebetween the optical density at the exposed area and the optical densityat the non-exposed area, and is preferably as high as possible. Thisenables the end-user to establish immediately whether or not theprecursor has already been exposed and processed, to distinguish thedifferent color selections and to inspect the quality of the image onthe plate precursor. The contrast of the print-out image preferablyincreases with increasing optical density in the exposed areas and canbe measured in reflectance using an optical densitometer, equipped withseveral filters (e.g. cyan, magenta, yellow).

The concentration of the thermochromic IR dyes with respect to the totaldry weight of the coating, may be from 0.1% wt to 20.0% wt, morepreferably from 0.5% wt to 15.0% wt, most preferred from 1.0% wt to10.0% wt.

The thermochromic IR dye is preferably represented by Formulae I, II orIII:

wherein

Ar¹, Ar² and Ar³ independently represent an optionally substitutedaromatic hydrocarbon group or an aromatic hydrocarbon group with anannulated benzene ring which is optionally substituted,

W¹ and W² independently represent a sulphur atom, an oxygen atom, NR″wherein R″ represents an optionally substituted alkyl group, NH, or a—CM¹⁰M¹¹ group wherein M¹⁰ and M¹¹ are independently an optionallysubstituted aliphatic hydrocarbon group or an optionally substituted(hetero)aryl group, or wherein M¹⁰ and M¹¹ together comprise thenecessary atoms to form a cyclic structure, preferably a 5- or6-membered ring;

W³ represent a sulphur atom or a —C(A³)=C(A⁴)-group,

W⁴ represents a sulphur atom or a —C(A⁷)=C(A⁸)-group,

M¹ and M² independently represent hydrogen, an optionally substitutedaliphatic hydrocarbon group or together comprise the necessary atoms toform an optionally substituted cyclic structure, preferably M¹ and M²together comprise the necessary atoms to form an optionally substitutedcyclic structure which may comprise an optionally substituted annulatedbenzene ring, preferably a 5- or 6-membered ring, more preferably a5-membered ring, most preferably a 5-membered ring having a cyclicstructure of 5 carbon atoms;

M³ and M⁴ independently represent an optionally substituted aliphatichydrocarbon group;

M⁵, M⁶, M⁷ and M⁸ , M¹⁶ and M¹⁷ independently represent hydrogen, ahalogen or an optionally substituted aliphatic hydrocarbon group,

A¹ to A⁸ independently represent hydrogen, a halogen atom, an optionallysubstituted aliphatic hydrocarbon group or an optionally substituted(hetero)aryl group, or wherein each of A¹ and A², A³ and A⁴, A⁵ and A⁶,or, A⁷ and A⁸, together comprise the necessary atoms to form a cyclicstructure, preferably 5- or 6-membered ring;

M¹² and M¹³ and M¹⁴ and M¹⁵ independently represent an optionallysubstituted aliphatic hydrocarbon group or an optionally substituted(hetero)aryl group, or wherein, two of said M¹⁴, M¹⁵, A⁵ or A⁷ togethercomprise the necessary atoms to form at least one cyclic structure,preferably 5- or 6-membered ring; two of said M¹², M¹³, A² or A⁴together comprise the necessary atoms to form at least one cyclicstructure preferably 5- or 6-membered ring;

M⁹ is a group which is transformed by a chemical reaction, induced byexposure to IR radiation or heat, into a group which is a strongerelectron-donor than said M⁹; and said transformation provides anincrease of the integrated light absorption of said dye between 350 and700 nm;

and optionally one or more counter ions in order to obtain anelectrically neutral compound.

The thermochromic IR dye can be a neutral, an anionic or a cationic dyedepending on the type of the substituting groups and the number of eachof the substituting groups. In a preferred embodiment, the dye offormula I, II or III comprises at least one anionic or acid group suchas —CO₂H, —CONHSO₂R^(h), —SO₂NHCOR^(i), —SO₂NHSO₂R^(i), —PO₃H₂, —OPO₃H₂,—OSO₃H, —S—SO₃H or —SO₃H groups or their corresponding salts, whereinR^(h), R^(i) and R^(j) are independently an aryl or an alkyl group,preferably a methyl group, and wherein the salts are preferably alkalimetal salts or ammonium salts, including mono- or di- or tri- ortetra-alkyl ammonium salts. These anionic or acid groups may be presenton the aromatic hydrocarbon group or the annulated benzene ring of Ar¹,Ar² or Ar³, or on the aliphatic hydrocarbon group of M³, M⁴ or M¹² toM¹⁵, or on the (hetero)aryl group of M¹² to M¹⁵. Other substitutinggroups can be selected from a halogen atom, a cyano group, a sulphonegroup, a carbonyl group or a carboxylic ester group.

In another preferred embodiment, at least one of M³, M⁴ or M¹² to M¹⁵ isterminally substituted with at least one of these groups, morepreferably with —CO₂H, —CONHSO₂—Me, —SO₂NHCO-Me, —SO₂NHSO₂—Me, —PO₃H₂ or—SO₃H groups or their corresponding salt, wherein Me represents a methylgroup.

In a preferred embodiment, the thermochromic IR dye represented byFormulae I, II or III above includes M⁹ represented by one of thefollowing groups:

—(N=CR¹⁷)a -NR⁵—CO—R⁴,

—(N=CR¹⁷)b -NR⁵—SO₂—R⁶,

—(N=CR¹⁷)c -NR¹¹—SO—R¹²,

—SO₂—NR¹⁵R¹⁶ and

—S—CH₂—CR⁷(H)_(1-d)(R⁸)_(d)—NR⁹—COOR¹⁸,

wherein

a, b, c and d independently are 0 or 1;

R¹⁷ represents hydrogen, an optionally substituted aliphatic hydrocarbongroup or an optionally substituted (hetero)aryl group, or wherein R¹⁷and R⁵ or R¹⁷ and R¹¹ together comprise the necessary atoms to form acyclic structure;

R⁴ represents —OR¹⁰, —NR¹³R¹⁴ or —CF₃;

wherein R¹⁰ represents an optionally substituted (hetero)aryl group oran optionally branched aliphatic hydrocarbon group;

R¹³ and R¹⁴ independently represent hydrogen, an optionally substitutedaliphatic hydrocarbon group or an optionally substituted (hetero)arylgroup, or wherein R¹³ and R¹⁴ together comprise the necessary atoms toform a cyclic structure;

R⁶ represents an optionally substituted aliphatic hydrocarbon group oran optionally substituted (hetero)aryl group, —OR¹⁰, —NR¹³R¹⁴ or —CF₃;

R⁵ represents hydrogen, an optionally substituted aliphatic hydrocarbongroup, a SO₃- group, a —COOR¹⁸ group or an optionally substituted(hetero)aryl group, or wherein R⁵ together with at least one of R¹⁰, R¹³and R¹⁴ comprise the necessary atoms to form a cyclic structure;

R11, R¹⁵ and R¹⁶ independently represent hydrogen, an optionallysubstituted aliphatic hydrocarbon group or an optionally substituted(hetero)aryl group, or wherein R¹⁵ and R¹⁶ together comprise thenecessary atoms to form a cyclic structure;

R¹² represents an optionally substituted aliphatic hydrocarbon group oran optionally substituted (hetero)aryl group;

R⁷ and R⁹ independently represent hydrogen or an optionally substitutedaliphatic hydrocarbon group;

R⁸ represents —COO— or —COOR^(8′) wherein R⁸′ represents hydrogen, analkali metal cation, an ammonium ion or a mono-, di-, tri- ortetra-alkyl ammonium ion;

R18 represents an optionally substituted (hetero)aryl group or analpha-branched aliphatic hydrocarbon group.

Suitable examples of thermochromic IR dyes used in the present inventionare described in EP 1 910 082 pages 4 to 8, IRD-001 to IRD-101, andincorporated herein by reference.

In a highly preferred embodiment, the thermochromic IR dye isrepresented by Formula IV

wherein Ar¹, Ar², W¹, W² and M¹ to M⁹ are as defined above.

Most preferably the thermochromic IR dye is represented by Formula IVwherein

Ar¹ and Ar² independently represent an optionally substituted arylgroup;

optionally annulated with an optionally substituted benzene ring,

W¹ and W² represent —C(CH₃)₂;

M¹ and M² together comprise the necessary atoms to form an optionallysubstituted 5-membered ring which may comprise an optionally substitutedannulated benzene ring;

M³ and M⁴ independently represent an optionally substituted aliphatichydrocarbon group,

M⁵, M⁶, M⁷ and M⁸ represent hydrogen;

M⁹ represents

—NR⁵—CO—R⁴

—NR⁵—SO₂—R⁶

—NR¹¹—SO—R¹²

—SO₂—NR¹⁵R¹⁶

wherein R⁴, R⁵, R⁶, R¹¹, R¹², R¹⁵, and R¹⁶ are as defined above;

and optionally one or more counter ions in order to obtain anelectrically neutral compound. Preferably the IR dye comprises at leastone anionic group or an acid group, such as —CO₂H, —CONHSO₂R^(h),—SO₂NHCOR^(i), —SO₂NHSO₂R^(j),—PO₃H₂, —OPO₃H₂, —OSO₃H, —SO₃H or —S—SO₃Hgroups or their corresponding salts, wherein R^(h), R^(i) and R^(j) areindependently an aryl or an alkyl group. More preferably, at least oneof the aliphatic hydrocarbon groups of M³ or M⁴ is terminallysubstituted with at least one of said anionic groups or acid groups.

In a highly preferred embodiment the thermochromic IR dye is representedby Formula I wherein

Ar¹ and Ar² independently represent an optionally substituted arylgroup;

W¹ and W² represent —C(CH₃)₂;

M¹ and M² together comprise the necessary atoms to form an optionallysubstituted 5-membered ring which may comprise an optionally substitutedannulated benzene ring;

M³ and M⁴ independently represent an optionally substituted aliphatichydrocarbon group,

M⁵, M⁶, M⁷ and M⁸ represent hydrogen;

M⁹ represents

—NR⁵—CO—R⁴

—NR⁵—SO₂—R⁶

wherein

R⁴ is —OR¹⁰, wherein R¹⁰ is an optionally branched aliphatic hydrocarbongroup;

R⁵ represents hydrogen, an optionally substituted aliphatic hydrocarbongroup or an optionally substituted (hetero)aryl group,

R⁶ represents an optionally substituted aliphatic hydrocarbon group oran optionally substituted (hetero)aryl group; and

optionally one or more counter ions in order to obtain an electricallyneutral compound.

Preferably the IR dye comprises at least one anionic group or an acidgroup, such as —CO₂H, —CONHSO₂R^(h), —SO₂NHCOR^(i), —SO₂NHSO₂R^(i),—PO₃H₂, —OPO₃H₂, —OSO₃H, —SO₃H or —S—SO₃H groups or their correspondingsalts, wherein R^(h), R^(i) and R^(j) are independently an aryl or analkyl group. More preferably, at least one of the aliphatic hydrocarbongroups of M³ or M⁴ is terminally substituted with at least one of saidanionic groups or acid groups. The salts are preferably alkali metalsalts or ammonium salts, including mono- or di- or tri- or tetra-alkylammonium salts.

The optional counter ions in order to obtain an electrically neutralcompound may be selected from for example a halogen, a sulphonate, aperfluorosulphonate, a tosylate, a tetrafluoroborate, ahexafluorophosphate, an arylborate, an arylsulphonate; or a cation suchas alkali metal salts or ammonium salts, including mono- or di- or tri-or tetra-alkyl ammonium salts.

Especially preferred thermochromic IR dye are presented by one of thefollowing formulae V to XII:

-   -   wherein    -   X⁻ represents halogen, sulphonate, perfluorosulphonate,        tosylate, tetrafluoroborate, hexafluorophosphate, arylborate or        arylsuiphonate; and    -   R³, R^(3′) independently represent an optionally substituted        alkyl group, preferably a methyl or ethyl; or an ether group,        preferably —CH2—CH₂—O—CH₃;

wherein

M⁺=Li⁺, Na⁺, K⁺, NH₄ ⁺, R′R″R′″NH⁺ wherein R′, R″, R′″ independentlyrepresent hydrogen, an optional substituted alkyl or aryl group;

The thermochromic IR dyes mentioned above may also be coupled to eachother or to other IR dyes as to form IR dye dimers or oligomers.

Besides a covalent coupling between two or more thermochromic IR dyes,supra-molecular complexes, comprising two or more thermochromic IR dyes,may also be formed by ionic interactions. Dimers, consisting of twodifferent IR dyes, may be formed for example by an interaction between acationic and an anionic IR dye, as described in e.g. WO/2004069938 andEP 1 466 728. IR dyes may also be ionically bond to a polymer as e.g.described in EP 1 582 346 wherein IR dyes, comprising two to foursulphonate groups are ionically bonded to a polymer comprisingcovalently attached ammonium, phosphonium, and sulphonium groups.

Supra-molecular complexes comprising two or more thermochromic R dyes,may also be formed by hydrogen bonding or dipole-dipole interaction.

The colour difference between the exposed and non-exposed areas of thecoating calculated from the L*a*b* values of the image areas (exposedareas) of the coating and the L*a*b* values of non-image areas(non-exposed areas) of the coating, is denoted as ΔE. Upon exposure ofthe coating of the present invention even with a low energy density, forexample between 70 and 190 mJ/m², more preferably between 75 and 150mJ/m², most preferably between 80 and 120 mJ/m², a print-out image isformed characterised by a CIE 1976 colour difference ΔE of at least 2,more preferably at least 2,5 and most preferably at least 3. Accordingto the present invention, a CIE 1976 colour difference ΔE of at least 2is obtained at very low exposure energies, for example below 150 mJ/m².ΔE is the CIE 1976 colour distance Delta E that is defined by the pairwise Euclidean distance of the CIE L*a*b* colour coordinates. CIE L*a*b*colour coordinates are obtained from reflection measurement in 45/0geometry (non-polarized), using CIE 2° observer and D50 as illuminant.More details are described in CIE S 014-4/E: 2007 Colourimetry—Part 4:CIE 1976 L*a*b* Colour Spaces and CIE publications and CIE S014-1/E:2006, CIE Standard Colourimetric Observers.

The CIE 1976 colour coordinates L*, a* and b* discussed herein are partof the well-known CIE (Commission Internationale de I'Eclairage) systemof tristimulus colour coordinates, which also includes the additionalchroma value C* defined as C*=[(a)²+(b)²]^(1/2). The CIE 1976 coloursystem is described in e.g. “Colorimetry, CIE 116-1995: IndustrialColour Difference Evaluation”, or in “Measuring Colour” by R. W. G.Hunt, second edition, edited in 1992 by Ellis Horwood Limited, England.

CIE L*a*b* values discussed and reported herein have been measuredfollowing the ASTM E308-85 method.

The toplayer may further include a binder. Preferred binders which canbe used in the toplayer are polyvinyl alcohol.The polyvinylalcohol haspreferably a hydrolysis degree ranging between 74 mol % and 99 mol %,more preferably between 80-98%. The weight average molecular weight ofthe polyvinylalcohol can be measured by the viscosity of an aqueoussolution, 4% by weight, at 20° C. as defined in DIN 53 015, and thisviscosity number ranges preferably between 2 and 26, more preferablybetween 2 and 15, most preferably between 2 and 10.

The overcoat layer may optionally include other ingredients such asinorganic or organic acids, matting agents, surfactants, (organic) waxesor wetting agents as disclosed in EP 2 916 171 and are incorporatedherein by reference.

The coating thickness of the toplayer is between 0.10 and 1.75 g/m²,preferably between 0.20 and 1.3 g/m², more preferably between 0.25 and1.0 g/m² and most preferably between 0.30 and 0.8 g/m². In a morepreferred embodiment of the present invention, the toplayer has acoating thickness between 0.25 and 1.75 g/m² and comprises apolyvinylalcohol having a hydrolysis degree ranging between 74 mol % and99 mol % and a viscosity number as defined above ranging between 3 and26.

The hydrophilic polymers in the protective overcoat layer may result ina problematic viscosity increase of press chemicals such as for examplefountain solution and/or developer solution. Therefore, the thickness ofthe protective overcoat layer should preferably not be too high e.g.above the ranges as given above.

Borate Compound

The coating of the present invention further includes at least oneborate compound. The borate compound is preferably present inphotopolymerisable layer. The borate compound present in the coatingrefers to a chemical compound including a borate anion and preferably acation as counterion. Preferably, the borate anion is a tetrahedralboron anion. The borate compound is preferably represented by thefollowing Formula A:

wherein R_(b) ¹, R_(b) ², R_(b) ³ and R_(b) ⁴ are independently anoptionally substituted aliphatic hydrocarbon group, an optionallysubstituted aryl or heteroaryl group; alternatively, two or more ofR_(b) ¹, R_(b) ², R_(b) ³ and R_(b) ⁴ can be joined together to form aheterocyclic ring with the boron atom, such a ring may include up toseven carbon, nitrogen, oxygen and/or nitrogen atoms; and M⁺ is analkali metal cation such as e.g. Li⁺, Na⁺, K⁺ or an optional substitutedonium ion.

In a preferred embodiment, R_(b) ¹, R_(b) ², R_(b) ³ and R_(b) ⁴ areindependently an optionally substituted aryl or heteroaryl group. Morepreferably, R_(b) ¹, R_(b) ², R_(b) ³ and R_(b) ⁴ are independently anoptionally substituted aryl group.

In a highly preferred embodiment the borate compound includes at leastone optionally substituted phenyl group, more preferably at least twooptionally substituted phenyl groups, even more preferably at leastthree optionally substituted phenyl groups and most preferably fouroptionally substituted phenyl groups.

Examples of the optionally substituted onium ion include pyridinium,ammonium, iodonium or sulfonium.

Examples of a pyridinium ion include N-alkyl-3-pyridinium group, anN-benzyl-3-pyridinium group, an N-(alkoxy polyalkyleneoxyalkyl)-3-pyridinium group, an N-alkoxycarbonylmethyl-3-pyridinium group,an N-alkyl -4pyridinium group, an N-benzyl-4-pyridinium group, anN-(alkoxy polyalkyleneoxy alkyl)-4-pyridinium group, anN-alkoxycarbonylmethyl-4-pyridinium group,N-alkyl-3,5-dimethyl-4-pyridinium, N-alkyl-3-pyridinium group orN-alkyl-4-pyridinium, an N-methyl-3-pyridinium, an N-octyl-3pyridinium,an N-methyl-4-pyridinium, or an N-octyl-4-pyridinium is particularlypreferred, and an Noctyl-3-pyridinium group or an N-octyl-4-pyridiniumgroup is most preferred.

The optional substituted onium ion is preferably an ammonium ionrepresented by Formula B:

wherein

R_(n) ¹,R_(n) ² and R_(n) ³ are independently an optionally substitutedaliphatic hydrocarbon group, an optionally substituted aryl orheteroaryl group or a halogen atom.

The optional substituted onium ion is most preferably a iodonium ion;more preferably an optionally substituted dipenyl iodonium salt.Diphenyl iodonium salts substituted with electron-donating groups, forexample, alkyl groups or alkoxyl groups, and asymmetric diphenyliodonium salts are particularly preferred.

In a preferred embodiment, the borate anion present in thephotopolymerisable layer originates from the counterion of thephotoinitiator (as described below).

Specific examples of borate compounds including a iodonium ion include4-hexyloxyphenyl-2,4-diethoxyphenyl iodonium tetrafluoroborate,4-octyloxyphenyl phenyliodonium tetraphenylborate,[4-[(2-hydroxytetradecyl) -oxy ]phenyl]phenyliodonium tetraphenylborate,bis(4-t-butylphenyl)iodonium tetraphenylborate, 4-methylphenyl-4′-hexylphenyliodoniumtetraphenylborate˜4-methylphenyl-4′-cyclohexylphenyliodoniumtetraphenylborate, bis(t-butylphenyl)iodoniumtetrakis(pentafluorophenyl)borate, 4-hexylphenyl-phenyliodoniumtetraphenylborate, 4-methylphenyl-4′-cyclohexylphenyliodoniumn-butyltriphenylborate, 4-cyclohexylphenyl-phenyliodoniumtetraphenylborate, 2-5 methyl-4-t-butylphenyl-4′-methylphenyliodoniumtetraphenylborate, 4-methylphenyl-4′-pentylphenyliodoniumtetrakis[3,5-bis(trifluoromethyl)phenyl]borate,4-methoxyphenyl-4′-cyclohexylphenyliodonium tetrakis(pentafluorophenyl)borate, 4-methylphenyl -4′-dodecylphenyliodonium tetrakis(4-fluorophenyl)borate, bis(dodecylphenyl)iodoniumtetrakis(pentatluorophenyl)borate, and bis(4-t-butylphenyl)iodoniumtetrakis(limidazolyl) borate. Preferred compounds includebis(4-t-butylphenyl)iodonium tetraphenylborate,4-methylphenyl-4′-hexyiphenyliodonium tetraphenylborate, 2-methyl-4-t-butylphenyl-4′-methylphenyliodonium tetraphenylborate, and 4-methylphenyl-4′-cyclohexylphenyliodonium tetraphenylborate.

In a preferred embodiment M⁺is an alkali metal cation such as e.g. Li⁺,Na⁺, K⁺, or an optional substituted ammonium ion according to Formula Bor an optionally substituted diphenyl iodonium ion. More preferably,M⁺is preferably an alkali metal cation such as e.g. Li⁺, Na⁺, K⁺ or anoptionally substituted diphenyl iodonium ion; most preferably anoptionally substituted diphenyl iodonium ion. The phenyl groups of theiodonium ion are preferably substituted with a group including at leastsix carbon atoms.

The borate compound is preferably present in an amount comprised between0.05 and 30% by weight, more preferably between 0.1 and 25% by weight,and most preferably from 0.5 and 15% by weight relative to thecomponents of the photopolymerisable layer.

Definitions

An aliphatic hydrocarbon group preferably represents an alkyl,cycloalkyl, alkenyl, cyclo alkenyl, allyl, or alkynyl group; suitablegroups thereof are described below. An aromatic hydrocarbon grouppreferably represents a hetero(aryl) group; suitable hetero(aryl)groups—i.e. suitable aryl or heteroaryl groups—are described below.

The term “alkyl” herein means all variants possible for each number ofcarbon atoms in the alkyl group i.e. methyl, ethyl, for three carbonatoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyland tertiary-butyl; for five carbon atoms: n-pentyl,1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, etc.Examples of suitable alkyl groups are methyl, ethyl, n-propyl,isopropyl, n-butyl, 1-isobutyl, 2-isobutyl and tertiary-butyl, n-pentyl,n-hexyl, chloromethyl, trichloromethyl, iso-propyl, iso-butyl,iso-pentyl, neo-pentyl, 1-methylbutyl and iso-hexyl,1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl and methylcyclohexyl groups.Preferably, the alkyl group is a C₁ to C6-alkyl group.

A suitable alkenyl group is preferably a C₂ to C6-alkenyl group such asan ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, iso-propenyl,iso-butenyl, iso-pentenyl, neo-pentenyl, 1-methylbutenyl, iso-hexenyl,cyclopentenyl, cyclohexenyl and methylcyclohexenyl group.

A suitable alkynyl group is preferably a C₂ to C₆-alkynyl group; asuitable aralkyl group is preferably a phenyl group or naphthyl groupincluding one, two, three or more C₁ to C6-alkyl groups; a suitablealkaryl group is preferably a C₁ to C₆-alkyl group including an arylgroup, preferably a phenyl group or naphthyl group.

A cyclic group or cyclic structure includes at least one ring structureand may be a monocyclic- or polycyclic group, meaning one or more ringsfused together.

Examples of suitable aryl groups may be represented by for example anoptionally substituted phenyl, benzyl, tolyl or an ortho- meta- orpara-xylyl group, an optionally substituted naphtyl, anthracenyl,phenanthrenyl, and/or combinations thereof. The heteroaryl group ispreferably a monocyclic or polycyclic aromatic ring comprising carbonatoms and one or more heteroatoms in the ring structure, preferably, 1to 4 heteroatoms, independently selected from nitrogen, oxygen, seleniumand sulphur. Preferred examples thereof include an optionallysubstituted furyl, pyridinyl, pyrimidyl, pyrazoyl, imidazoyl, oxazoyl,isoxazoyl, thienyl, tetrazoyl, thiazoyl, (1,2,3)triazoyl,(1,2,4)triazoyl, thiadiazoyl, thiofenyl group and/or combinationsthereof.

A cyclic group or cyclic structure includes at least one ring structureand may be a monocyclic- or polycyclic group, meaning one or more ringsfused together.

Halogens are selected from fluorine, chlorine, bromine or iodine.

The term “substituted”, in e.g. substituted alkyl group means that thealkyl group may be substituted by other atoms than the atoms normallypresent in such a group, i.e. carbon and hydrogen. For example, asubstituted alkyl group may include a halogen atom or a thiol group. Anunsubstituted alkyl group contains only carbon and hydrogen atoms.

The optional substituents on the alkyl, cycloalkyl, alkenyl,cycloalkenyl, alkynyl, aralkyl, alkaryl, aryl and heteroaryl group arepreferably selected from —Cl, —Br, —I, —OH, —SH, —CN, —NO₂, an alkylgroup such as a methyl or ethyl group, an alkoxy group such as a methoxyor an ethoxy group, an aryloxy group, a carboxylic acid group or analkyl ester thereof, a sulphonic acid group or an alkyl ester thereof, aphosphonic acid group or an alkyl ester thereof, a phosphoric acid groupor an an ester such as an alkyl ester such as methyl ester or ethylester, a thioalkyl group, a thioaryl group, thioheteroaryl, —SH, athioether such as a thioalkyl or thioaryl, ketone, aldehyde, sulfoxide,sulfone, sulfonate ester, sulphonamide, an amino, ethenyl, alkenyl,alkynyl, cycloalkyl, alkaryl, aralkyl, aryl, heteroaryl orheteroalicyclic group and/or combinations thereof.

Support

The lithographic printing plate used in the present invention comprisesa support which has a hydrophilic surface or which is provided with ahydrophilic layer. The support is preferably a grained and anodizedaluminium support, well known in the art. Suitable supports are forexample disclosed in EP 1 843 203 (paragraphs [0066] to [0075]). Thesurface roughness, obtained after the graining step, is often expressedas arithmetical mean center-line roughness Ra (ISO 4287/1 or DIN 4762)and may vary between 0.05 and 1.5 μm. The aluminum substrate of thecurrent invention has preferably an Ra value between 0.1 and 1.4 μm,more preferably between 0.3 and 1.0 μm and most preferably between 0.4and 0.9 μm. The lower limit of the Ra value is preferably about 0.1 μm.More details concerning the preferred Ra values of the surface of thegrained and anodized aluminum support are described in EP 1 356 926. Byanodising the aluminum support, an Al₂O₃ layer is formed and the anodicweight (g/m² Al₂O₃ formed on the aluminum surface) varies between 1 and8 g/m². The anodic weight is preferably >2.0 g/m², more preferably 2.5g/m² and most preferably ≥3.0 g/m²

The grained and anodized aluminium support may be subjected to so-calledpost-anodic treatments, for example a treatment with polyvinylphosphonicacid or derivatives thereof, a treatment with polyacrylic acid orderivatives thereof, a treatment with potassium fluorozirconate or aphosphate, a treatment with an alkali metal silicate, or combinationsthereof. Enlargement or sealing of micropores of the amodized aluminumas disclosed in JP2001-253181A or JP2001-322365A may be performed.Alternatively, the support may be treated with an adhesion promotingcompound such as those described in EP 1 788 434 in [0010] and in WO2013/182328. However, for a precursor optimized to be used without apre-heat step it is preferred to use a grained and anodized aluminiumsupport without any post-anodic treatment.

Besides an aluminium support, a plastic support, for example a polyestersupport, provided with one or more hydrophilic layers as disclosed infor example EP 1 025 992 may also be used.

Photopolymer Coating Photo Olymerisable Compound

The coating has at least one layer including a photopolymerisablecomposition, said layer is also referred to as the “photopolymerisablelayer”. The coating may include an intermediate layer, located betweenthe support and the photopolymerisable layer.

The photopolymerisable layer includes at least one polymerisablecompound, an initiator and optionally a binder. The photopolymerisablelayer has a coating thickness preferably ranging between 0.2 and 5.0g/m², more preferably between 0.4 and 3.0 g/m², most preferably between0.6 and 2.2 g/m².

According to a preferred embodiment of the present invention, thepolymerisable compound is a polymerisable monomer or oligomer includingat least one terminal ethylenic unsaturated group, hereinafter alsoreferred to as “free-radical polymerisable monomer”. The polymerisationinvolves the linking together of the free-radical polymerisablemonomers. Suitable free-radical polymerisable monomers include, forexample, multifunctional (meth)acrylate monomers (such as (meth)acrylateesters of ethylene glycol, trimethyloipropane, pentaerythritol, ethyleneglycol, ethoxylated trimethyloipropane, urethane (meth)acrylate) andoligomeric amine di(meth)acrylates. The (meth)acrylic monomers may alsohave other ethylenically unsaturated groups or epoxide groups inaddition to the (meth)acrylate group. The (meth)acrylate monomers mayalso contain an acidic (such as a carboxylic acid or phosphoric acid) orbasic (such as an amine) functionality.

Suitable free-radical polymerisable monomers are disclosed in [0042] and[0050] of EP 2 916 171 and are incorporated herein by reference.

The Initiator

Any free radical initiator capable of generating free radicals uponexposure directly or in the presence of a sensitizer, is according tothis invention a suitable initiator; also referred to as photoinitiator.Suitable examples of initiators include onium salts, carbon-halogenbond-containing compounds such as [1,3,5] triazines having trihalomethylgroups, organic peroxides, aromatic ketones, thio compounds, azo basedpolymerization initiators, azide compounds, ketooxime esters,hexaarylbisimidazoles, metallocenes, active ester compounds, borates andquinonediazides. Of these, onium salts, especially iodonium and/orsulfonium salts are preferable in view of storage stability.

More specific suitable free-radical initiators include, for example, thederivatives of acetophenone (such as 2,2-dimethoxy-2-phenylacetophenone,and 2-methyl-l-[4-(methylthio) phenyll-2-morpholino propan-l-one);benzophenone; benzil; ketocoumarin (such as 3-benzoyl-7-methoxy coumarinand 7-methoxy coumarin); xanthone; thioxanthone; benzoin or analkyl-substituted anthraquinone; onium salts (such as diaryliodoniumhexafluoroantimonate, diaryliodonium triflate, (4-(2-hydroxytetradecyl-oxy)-phenyl) phenyliodonium hexafluoroantimonate, triarylsulfoniumhexafluorophosphate, triarylsulfonium p-toluenesulfonate,(3-phenylpropan-2-onyl) triaryl phosphonium hexafluoroantimonate, andN-ethoxy(2-methyl)pyridinium hexafluorophosphate, and onium salts asdescribed in U.S. Pat. Nos. 5,955,238, 6,037,098, and 5,629,354); boratesalts (such as tetrabutylammonium triphenyl(n-butyl)borate,tetraethylammonium triphenyl(n-butyl)borate, diphenyliodoniumtetraphenylborate, diphenyliodonium tetraphenylborate wherein the phenylgroups of the iodonium salt are substituted with a group including atleast six carbon atoms, and triphenylsulfonium triphenyl(n-butyl)borate,and borate salts as described in U.S. Pat. Nos. 6,232,038 and6,218,076,); haloalkyl substituted s-triazines (such as2,4-bis(trichloromethyl)-6-(p-methoxy -styryl)-s-triazine,2,4-bis(trichloromethyl)-6-(4-methoxy-naphth-l-yl) -s-triazine,2,4-bis(trichloromethyl)-6-piperonyl-s- triazine, and2,4-bis(trichloromethyl) -6-[(4-ethoxy-ethylenoxy)-phen-1-yl]-s-triazine, and s-triazines as describedin U.S. Pat. Nos. 5,955,238, 6,037,098, 6,010,824 and 5,629,354); andtitanocene (bis(etha.9-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(IH-pyrrol-1-yl)phenyl) titanium). Onium salts,borate salts, and s-triazines are preferred free radical initiators.Diaryliodonium salts and triarylsulfonium salts are preferred oniumsalts. Triarylalkylborate salts are preferred borate salts.Trichloromethyl substituted s-triazines are preferred s-triazines. Theseinitiators may have optional substituents and may be used alone or incombination.

Optionally substituted trihaloalkyl sulfones wherein halo independentlyrepresents bromo, chloro or iodo and sulfone is a chemical compoundcontaining a sulfonyl functional group attached to two carbon atoms, areparticularly preferred initiators. Tribromomethyl phenyl sulfones aremost preferred initiators. More details concerning this initiator can befound in unpublished copending application EP 18163285.2 paragraphs[0029] to [0040].

The amount of the initiator typically ranges from 0.1 to 30% by weight,preferably from 0.5 to 15% by weight, most preferably from 2 to 10% byweight relative to the total dry weight of the components of thephotopolymerisable composition.

A very high sensitivity can be obtained by the combination of an opticalbrightener as sensitizer and a polymerisation initiator.

The photopolymerisable layer may also comprise a co-initiator.Typically, a co-initiator is used in combination with a free radicalinitiator. Suitable co-initiators for use in the photopolymer coatingare disclosed in U.S. Pat. Nos. 6,410,205; 5,049,479; EP 1 079 276, EP 1369 232, EP 1 369 231, EP 1 341 040, US 2003/0124460, EP 1 241 002, EP 1288 720 and in the reference book including the cited refences:Chemistry & Technology UV & EB formulation for coatings, inks &paints—Volume 3—Photoinitiators for Free Radical and CationicPolymerisation by K.K. Dietliker—Edited by P.K.T. Oldring—1991—ISBN 0947798161. Specific co-initiators, as described in EP 107 792, may bepresent in the photopolymerizable layer to further increase thesensitivity. Preferred co-initiators are disclosed in EP 2 916 171[0051] and are incorporated herein by reference.

A very high sensitivity can be obtained by including a sensitizer suchas for example an optical brightener in the coating. Suitable examplesof optical brighteners as sensitizers are described in WO 2005/109103page 24, line 20 to page 39. Useful sensitizers can be selected from thesensitizing dyes disclosed in U.S. Pat. Nos. 6,410,205; 5,049,479; EP 1079 276, EP1 369 232, EP 1 369 231, EP 1 341 040, US 2003/0124460, EP 1241 002 and EP 1 288 720.

Specific co-initiators, as described in EP 107 792, may be present inthe photopolymerizable layer to further increase the sensitivity.Preferred co-initiators are sulfur-compounds, especially thiols likee.g. 2-mercaptobenzothiazole, 2-mercaptobenzoxazole,2-mercapto-benzimidazole, 4-methyl-3-propyl-1,2,4-triazoline-5-thione,4-methyl-3-n-heptyl -1,2,4-triazoline-5-thione,4-phenyl-3-n-heptyl-1,2,4-triazoline-5-thione,4-phenyl-3,5-dimercapto-1,2,4-triazole,4-n-decyl-3,5-dimercapto-1,2,4-triazole,5-phenyl-2-mercapto-1,3,4-oxadiazole, 5-methylthio-1,3,4-thiadiazoline-2-thione, 5-hexylthio-1,3,4-thiadiazoline-2-thione,mercaptophenyltetrazole, pentaerythritol mercaptopropionate, butyricacid-3-mercapto-neopentanetetrayl ester, pentaerythritoltetra(thioglycolate). Other preferred co-initiators are polythioles asdisclosed in WO 2006/048443 and WO 2006/048445. These polythiols may beused in combination with the above described thiols, e.g.2-mercaptobenzothiazole.

The photopolymerizable layer may optionally include infrared lightabsorbing dyes as sensitizers absorbing light between 750 nm and 1300nm, preferably between 780 nm and 1200 nm, more preferably between 800nm and 1100 nm. Particular preferred sensitizers are heptamethinecyaninedyes disclosed in EP 1 359 008 paragraph [0030] to [0032].

The Binder

The photopolymerizable layer preferably includes a binder. The bindercan be selected from a wide series of organic polymers. Compositions ofdifferent binders can also be used. Useful binders are described in forexample EP 1 043 627 in paragraph [0013]. Also particulate shapedpolymers including homopolymers or copolymers prepared from monomerssuch as ethylene, styrene, vinyl chloride, methyl acrylate, ethylacrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile, vinylcarbazole, acrylate or methacrylate, or mixtures thereof.

Thermally reactive polymer fine particles including a thermally reactivegroup such as an ethylenically unsaturated group, a cationicpolymerizable group, an isocyanate group, an epoxy group, a vinyloxygroup, and a functional group having an active hydrogen atom, a carboxygroup, a hydroxy group, an amino group or an acid anhydride.

The average particle diameter of the polymer fine particle is preferably0.01 mm to 3.0 mm. Particulate polymers in the form of microcapsules,microgels or reactive microgels are suitable as disclosed in EP 1 132200; EP 1 724 112; US 2004/106060.

Other Ingredients

The photopolymerisable layer may also comprise particles which increasethe resistance of the coating against manual or mechanical damage. Theparticles may be inorganic particles, organic particles or fillers suchas described in for example U.S. Pat. No. 7,108,956. More details ofsuitable spacer particles described in EP 2 916 171 [0053] to [0056] areincorporated herein by reference.

The photopolymerizable layer may also comprise an inhibitor. Particularinhibitors for use in the photopolymer coating are disclosed in U.S.Pat. No. 6,410,205, EP 1 288 720 and EP 1 749 240.

The photopolymerizable layer may further comprise an adhesion promotingcompound. The adhesion promoting compound is a compound capable ofinteracting with the support, preferably a compound having anaddition-polymerizable ethylenically unsaturated bond and a functionalgroup capable of interacting with the support. Under “interacting” isunderstood each type of physical and/or chemical reaction or processwhereby, between the functional group and the support, a bond is formedwhich can be a covalent bond, an ionic bond, a complex bond, acoordinate bond or a hydrogen-bond, and which can be formed by anadsorption process, a chemical reaction, an acid-base reaction, acomplex-forming reaction or a reaction of a chelating group or a ligand.The adhesion promoting compounds described in EP 2 916 171 [0058] areincorporated herein by reference.

Various surfactants may be added into the photopolymerisable layer toallow or enhance the developability of the precursor; especiallydeveloping with a gum solution. Both polymeric and small moleculesurfactants for example nonionic surfactants are preferred. More detailsare described in EP 2 916 171 [0059] and are incorporated herein byreference.

The lithographic printing plate precursor according to the presentinvention can be prepared by (i) applying on a support the coating asdescribed above and (ii) drying the precursor.

Exposure Step

The printing plate precursor is preferably image-wise exposed by a laseremitting IR light. Preferably, the image-wise exposing step is carriedout off-press in a platesetter, i.e. an exposure apparatus suitable forimage-wise exposing the precursor with a laser such as a laser diode,emitting around 830 nm or a Nd YAG laser emitting around 1060 nm, or bya conventional exposure in contact with a mask. In a preferredembodiment of the present invention, the precursor is image-wise exposedby a laser emitting IR light.

Preheat Step

After the exposing step, the precursor may be pre-heated in a preheatingunit, preferably at a temperature of about 80° C. to 150° C. andpreferably during a dwell time of about 5 seconds to 1 minute. Thispreheating unit may comprise a heating element, preferably an IR-lamp,an UV-lamp, heated air or a heated roll. Such a preheat step can be usedfor printing plate precursors comprising a photopolymerisablecomposition to enhance or to speed-up the polymerization and/orcrosslinking reaction.

Development Step

Subsequently to the exposing step or the preheat step, when a preheatstep is present, the plate precursor may be processed (developed).Before developing the imaged precursor, a pre-rinse step might becarried out especially for the negative-working lithographic printingprecursors having a protective oxygen barrier or topcoat. This pre-rinsestep can be carried out in a stand-alone apparatus or by manuallyrinsing the imaged precursor with water or the pre-rinse step can becarried out in a washing unit that is integrated in a processor used fordeveloping the imaged precursor. The washing liquid is preferably water,more preferably tap water. More details concerning the wash step aredescribed in EP 1 788 434 in [0026].

During the development step, the non-exposed areas of theimage-recording layer are at least partially removed without essentiallyremoving the exposed areas. The processing liquid, also referred to asdeveloper, can be applied to the plate e.g. by rubbing with animpregnated pad, by dipping, immersing, coating, spincoating, spraying,pouring-on, either by hand or in an automatic processing apparatus. Thetreatment with a processing liquid may be combined with mechanicalrubbing, e.g. by a rotating brush. During the development step, anywater-soluble protective layer present is preferably also removed. Thedevelopment is preferably carried out at temperatures between 20 and 40°C. in automated processing units.

In a highly preferred embodiment, the processing step as described aboveis replaced by an on-press processing whereby the imaged precursor ismounted on a press and processed on-press by rotating said platecylinder while feeding dampening liquid and/or ink to the coating of theprecursor to remove the unexposed areas from the support. In a preferredembodiment, only dampening liquid is supplied to the plate duringstart-up of the press and after a number of revolutions of the platecylinder also the ink supply is switched on. In an alternativeembodiment, supply of dampening liquid and ink is startedsimultaneously, or only ink can be supplied during a number ofrevolutions before switching on the supply of dampening liquid.

The processing step may also be performed by combining embodimentsdescribed above, e.g. combining development with a processing liquidwith development on-press by applying ink and/or fountain.

Processing Liquid

The processing liquid may be an alkaline developer or solvent-baseddeveloper. Suitable alkaline developers have been described inUS2005/0162505. An alkaline developer is an aqueous solution which has apH of at least 11, more typically at least 12, preferably from 12 to 14.Alkaline developers typically contain alkaline agents to obtain high pHvalues can be inorganic or organic alkaline agents. The developers cancomprise anionic, non-ionic and amphoteric surfactants (up to 3% on thetotal composition weight); biocides (antimicrobial and/or antifungalagents), antifoaming agents or chelating agents (such as alkaligluconates), and thickening agents (water soluble or water dispersiblepolyhydroxy compounds such as glycerine or polyethylene glycol).

Preferably, the processing liquid is a gum solution whereby during thedevelopment step the non-exposed areas of the photopolymerisable layerare removed from the support and the plate is gummed in a single step.The development with a gum solution has the additional benefit that, dueto the remaining gum on the plate in the non-exposed areas, anadditional gumming step is not required to protect the surface of thesupport in the non-printing areas. As a result, the precursor isprocessed and gummed in one single step which involves a less complexdeveloping apparatus than a developing apparatus comprising a developertank, a rinsing section and a gumming section. The gumming section maycomprise at least one gumming unit or may comprise two or more gummingunits. These gumming units may have the configuration of a cascadesystem, i.e. the gum solution, used in the second gumming unit andpresent in the second tank, overflows from the second tank to the firsttank when gum replenishing solution is added in the second gumming unitor when the gum solution in the second gumming unit is used once-only,i.e. only starting gum solution is used to develop the precursor in thissecond gumming unit by preferably a spraying or jetting technique. Moredetails concerning such gum development is described in EP1 788 444.

A gum solution is typically an aqueous liquid which comprises one ormore surface protective compounds that are capable of protecting thelithographic image of a printing plate against contamination, e.g. byoxidation, fingerprints, fats, oils or dust, or damaging, e.g. byscratches during handling of the plate. Suitable examples of suchsurface protective compounds are film-forming hydrophilic polymers orsurfactants. The layer that remains on the plate after treatment withthe gum solution preferably comprises between 0.005 and 20 g/m² of thesurface protective compound, more preferably between 0.010 and 10 g/m²,most preferably between 0.020 and 5 g/m². More details concerning thesurface protective compounds in the gum solution can be found in WO2007/057348 page 9 line 3 to page 11 line 6. As the developed plateprecursor is developed and gummed in one step, there is no need topost-treat the processed plate.

The gum solution preferably has a pH value between 3 and 11, morepreferably between 4 and 10, even more preferably between 5 and 9, andmost preferably between 6 and 8. A suitable gum solution is described infor example EP 1 342 568 in [0008] to [0022] and WO2005/111727. The gumsolution may further comprise an inorganic salt, an anionic surfactant,a wetting agent, a chelate compound, an antiseptic compound, ananti-foaming compound and/or an ink receptivity agent and/orcombinations thereof. More details about these additional ingredientsare described in WO 2007/057348 page 11 line 22 to page 14 line 19.

Drying and Baking Step

After the processing step the plate may be dried in a drying unit. In apreferred embodiment the plate is dried by heating the plate in thedrying unit which may contain at least one heating element selected froman IR-lamp, an UV-lamp, a heated metal roller or heated air.

After drying the plate can optionally be heated in a baking unit. Moredetails concerning the heating in a baking unit can be found in WO2007/057348 page 44 line 26 to page 45 line 20.

The printing plate thus obtained can be used for conventional, so-calledwet offset printing, in which ink and an aqueous dampening liquid issupplied to the plate. Another suitable printing method uses a so-calledsingle-fluid ink without a dampening liquid. Suitable single-fluid inkshave been described in U.S. Pat. Nos. 4,045,232; 4,981,517 and6,140,392. In a most preferred embodiment, the single-fluid inkcomprises an ink phase, also called the hydrophobic or oleophilic phase,and a polyol phase as described in WO 00/32705.

EXAMPLES Example 1 1. Preparation of the Printing Plate PrecursorsPreparation of the Aluminium Support S-01

A 0.3 mm thick aluminium foil was degreased by spraying with an aqueoussolution containing 26 g/I NaOH at 65° C. for 2 seconds and rinsed withdemineralised water for 1.5 seconds. The foil was then electrochemicallygrained during 10 seconds using an alternating current in an aqueoussolution containing 15 g/l HCl, 15 g/l SO42- ions and 5 g/l Al3+ ions ata temperature of 37° C. and a current density of about 100 A/dm2.Afterwards, the aluminium foil was then desmutted by etching with anaqueous solution containing 5.5 g/l of NaOH at 36° C. for 2 seconds andrinsed with demineralised water for 2 seconds. The foil was subsequentlysubjected to anodic oxidation during 15 seconds in an aqueous solutioncontaining 145 g/l of sulfuric acid at a temperature of 50° C. and acurrent density of 17 A/dm2, then washed with demineralised water for 11seconds and dried at 120° C. for 5 seconds.

The support thus obtained was characterized by a surface roughness Ra of0.35-0.4 μm (measured with interferometer NT1100) and had an oxideweight of 3.0 g/m².

Preparation of Inventive Printing Plates PP-01 to PP-08 and ComparativePrinting Plates PP-09 to PP-11 Photopolymerisable Layer

The photosensitive composition PL-01 as defined in Table 1 was coatedonto the above described support S-01. The components were dissolved ina mixture of 35% by volume of MEK and 65% by volume of Dowanol PM(1-methoxy-2-propanol, commercially available from DOW CHEMICALCompany). The coating solution was applied at a wet coating thickness of30 μm and then dried at 120° C. for 1 minute in a circulation oven.

TABLE 1 composition of the photosensitive layer Ingredients g/m² PL-01FST 510 (1) 0.250 CN 104 (2) 0.250 Initiator-01 (3) 0.045 S2025 (4)0.020 S-LEC BX35-Z (5) 0.150 Tegoglide 410 (6) 0.0015 Sipomer PAM 100(7) 0.130 Albritect CP 30 (8) 0.024 1) FST 510 is a reaction productfrom 1 mole of 2,2,4-trimethylhexamethylenediisocyanate and 2 moles ofhydroxyethyl-methacrylate commercially available from AZ Electronics asa 82 wt. % solution in MEK; 2) CN 104 is an epoxy acrylate oligomercommercially available from Arkema; 3) Initiator-01 isbis(4-tert-butylphenyl)-iodonium tetraphenyl borate 4) S2025 is aninfrared absorbing dye commercially available from FEW Chemicals

5) S-LEC BX35-Z, a polyvinylbutyral commercially available from Sekisui;6) Tegoglide 410 is a surfactant commercially available from Evonik TegoChemie GmbH; 7) Sipomer PAM 100 is a methacrylate phosphonic estercommercially available from Rhodia; 8) Albritect CP 30, is a copolymerof vinylphosphonic acid and acrylic acid commercially available as a 20wt. % aqueous dispersion from Rhodia.

Protective Overcoat Layer

On top of the photosensitive layer, a solution in water with thecompositions as defined in Tables 2 and 3 was coated (40 μm) on thephotosensitive layer, and dried at 110° C. for 2 minutes. Inventiveprinting plate precursors PPP-01 to PPP-08 having a coating thickness ofrespectively 1.225; 1.245; 1.285; 1.985; 0.512 and 0.552 g/m² wereobtained and comparative printing plate precursors PPP-09 to PPP-11having a coating thickness of respectively 1.205; 0.510 and 0.522 g/m²were obtained.

TABLE 2 composition of the inventive protective overcoat layers (g/m²)Ingredients OC-01 OC-02 OC-03 OC-04 OC-05 OC-06 OC-07 OC-08 g/m² Inv InvInv Inv Inv Inv Inv Inv Mowiol4-88 0.736 0.736 0.736 0.736 0.450 0.4500.450 0.450 (1) Mowiol8-88 0.226 0.226 0.226 0.226 — — — — (1) LuviskolK30 0.231 0.231 0.231 0.231 — — — — (2) IR-01 (3) 0.020 0.040 0.0800.150 0.020 0.050 — — IR-02 (3) — — — — — — 0.050 0.090 Lutensol A80.012 0.012 0.012 0.012 0.012 0.012 0.012 0.012 (4) 1) Mowiol 488 ™ andMowiol 888 ™ are partially hydroyzed polyvinylalcohols commerciallyavailable from Kuraray; 2) Luviskol K30 ™ is polyvinylpyrrolidonehomopolymer commercially available from BASF; 3) IR-01 is an infraredabsorbing dye having the following formula:

IR-02 is an infrared absorbing dye having the following formula:

4) Lutensol A8 ™ is a surface active agent commercially available fromBASF.

TABLE 3 composition of the comparative protectiveovercoat layersIngredients OC-09 OC-10 OC-11 g/m² reference comparative comparativeMowiol4-88 (1) 0.736 0.410 0.410 Mowiol8-88 (1) 0.226 — — Luviskol K30(2) 0.231 — — IR-03 (3) — 0.020 0.020 Black XV (4) — — 0.020 Omnicat 250(5) — 0.060 0.060 utensol A8 (6) 0.012 0.012 0.012 (1) (2) and (6) seeTable 2 3) IR-03 is an infrared absorbing dye commercially availablefrom FEW chemicals having the following formula:

4) Black XV is 6-diethylamino-3-methyl-2-(2,4-xylidino) fluoran, a leucodye commercially available from Mitsui; 5) Acid generator is(4-methylphenyl)[4-(2-methylpropyl)phenyl]-hexafluorophosphatecommercially available from IGM.

2. Imaging

The printing plate precursors PPP-01 to PPP-11 were imaged at 2400 dpiwith a High Power Creo 40W TE38 thermal platesetter™ (200 Ipi AgfaBalanced Screening (ABS)), commercially available from Kodak andequipped with a 830 nm IR laser diode, at energy densities between 50and 200 mJ/cm². Printing plates PP-01 to PP-11 were obtained.

3. ΔE Measurement

Lab measurement executed with a GretagMacBeth Spectro Eye reflectionspectrophotometer with the settings: D50 (illuminant), 2° (Observer), Nofilter; commercially available from GretagMacBeth. The total colourdifference ΔE is a single value that takes into account the differencebetween the L, a* and b* values of the image areas and the non-imageareas:

ΔE=√{square root over (ΔL ² +Δa ² +Δb ²)}

The higher the total colour difference ΔE, the better the obtainedcontrast.

The contrast between image and non-image areas results in the occurrenceof a print-out image.

4. Results

The total colour difference ΔE was measured and the results aresummarized in the Tables 4 to 7 below.

A total colour difference ΔE above or equal to 2 is defined as a clearprint out image. A total colour difference ΔE below 2 is defined as anunsufficient print out image.

4.1 Effect of the Concentration of the Dye in the OC

TABLE 4 Concentration of the IR-01 in the OC Printing Overcoat IR dyeExposure energy plate layer g/m² mJ/cm² ΔE* PP-01 OC-01 0.020 120 3.0inventive PP-02 OC-02 0.040 120 4.5 inventive PP-03 OC-03 0.080 120 8.2inventive PP-04 OC-04 0.150 120 14.5 inventive *see above

The results in Table 4 demonstrate that the print-out image is strong(the total colour difference ΔE is equal or higher than 2) for theinventive printing plates including an overcoat layer including the dyeaccording to the present invention. The results further show thatincreasing the level of IR dye according to the present invention in theovercoat layer, further improves the print-out image; i.e. the totalcolour difference ΔE increases with an increasing level of IR dyeaccording to the present invention.

4.2 Effect of the Exposure Energy

TABLE 5 Exposure energy Printing Exposure energy plate PP-04 mJ/cm² ΔE*OC-04 60 2.6 Inventive OC-04 120 14.5 Inventive OC-04 180 25.8 Inventive*see above

The results in Table 5 demonstrate that the print-out image is strong(the total colour difference ΔE is equal or higher than 2) for theinventive printing plates including an overcoat layer including the dyeaccording to the present invention. The results further show that theapplied exposure energy setting significantly affects the print-outimage; the higher the exposure energy, the higher ΔE.

4.3 Effect of the IR dye and/or Concentration

TABLE 6 IR dye and/or concentration Exposure Printing OvercoatConcentration energy plate layer IR dye g/m² mJ/cm² ΔE* PP-05 OC-05IR-01 0.02 150 3.9 Inventive PP-06 OC-06 IR-01 0.05 150 8.2 InventivePP-07 OC-07 IR-02 0.05 150 5.2 Inventive PP-08 OC-08 IR-02 0.09 150 11.2Inventive *see above

The results in Table 6 demonstrate that the print-out image isstrong—the total colour difference ΔE is equal or higher than 2—for theinventive printing plates including an overcoat layer including an IRdye according to the present invention.

4.4 Comparative Printing Plates

TABLE 7 Comparative Printing Plates Printing Overcoat Exposure energyplate layer (mJ/cm2) ΔE* PP-09 OC-09 120 1.3 comparative PP-10 OC-10 1201.5 comparative PP-11 OC-11 120 1.9 comparative

The results in Table 7 demonstrate that the comparative printing platesPP-09 to PP-11 show a weak print-out image (the total colour differenceΔE is lower than 2). Comparative printing plates PP-10 and PP-11 whichcontain an IR dye of the prior art, an acid generator and/or a IR leucodye of the prior art, result in a poor print-out image.

Example 2 1. Preparation of Comparative Printing Plate PP-12Photopolymerisable Layer

The photopolymerisable layer PL-02 as defined in Table 8 was coated ontothe above described support S-01 (see Example 1). The components weredissolved in a mixture of 35% by volume of MEK and 65% by volume ofDowanol PM (1-methoxy-2-propanol, commercially available from DOWCHEMICAL Company). The coating solution was applied at a wet coatingthickness of 30 μm and then dried at 120° C. for 1 minute in acirculation oven.

TABLE 8 composition of the photosensitive layer (g/m²) Ingredients g/m²PL-02 FST 510 (1) 0.250 CN 104 (2) 0.250 Initiator-02 (3) 0.060 S0094(4) 0.020 Ruco coat EC4811 (5) 0.250 Tegoglide 410 (6) 0.0015 SipomerPAM 100 (7) 0.130 Albritect CP 30 (8) 0.024 1) FST 510 is a reactionproduct from 1 mole of 2,2,4-trimethylhexamethylenediisocyanate and 2moles of hydroxyethyl-methacrylate commercially available from AZElectronics as a 82 wt. % solution in MEK; 2) CN 104 is an epoxyacrylate oligomer commercially available from Arkema; 3) Initiator-02 is4-hydroxyphenyl-tribromomethyl-sulfone 4) S0094 is an infrared absorbingdye commercially available from FEW Chemicals having the followingstructure:

5) Ruco coat EC4811 is a polyether polyurethane commercially availablefrom Rudolf Chemistry 6) Tegoglide 410 is a surfactant commerciallyavailable from Evonik Tego Chemie GmbH; 7) Sipomer PAM 100 is amethacrylate phosphonic ester commercially available from Rhodia; 8)Albritect CP 30, is a copolymer of vinylphosphonic acid and acrylic acidcommercially available as a 20 wt. % aqueous dispersion from Rhodia.

Protective Top layer

On top of the photosensitive layer, a solution in water with thecomposition as defined in Table 9 was coated on the photosensitivelayer, and dried at 110° C. for 2 minutes. Printing plate precursorPPP-12 having a coating thickness of 2 g/m² was obtained.

TABLE 9 composition of the protective layer (g/m²) Ingredients OC-12g/m² comp Mowiol 4-88 (1) 0.785 Mowiol 8-88 (1) 0.343 Mowiol 6-98 (1)0.685 IR-02(2) 0.158 Acticide LA1206 (3) 0.012 Metolat FC 355 (4) 0.018Lutensol A8 (5) 0.0015 1) Mowiol 4-88 ™, Mowiol 8-88 ™ are partiallyhydrolyzed polyvinylalcohols and Mowiol 6-98 ™ is a fully hydrolysedpolyvinylalcohol commercially available from Kuraray; 2) IR-02 is aninfrared absorbing dye having the following formula:

3) Acticide LA1206 is a biocide, commercially available from Thor. 4)Metolat FC 355 is an ethoxylated ethylenediamine, commercially availablefrom Munzing Chemie. 5) Lutensol A8 ™ is a surface active agent,commercially available from BASF.

2. Imaging

The printing plate precursor PPP-12 was imaged at 2400 dpi with a HighPower Creo 40W TE38 thermal platesetter™ (200 Ipi Agfa BalancedScreening (ABS)), commercially available from Kodak and equipped with a830 nm IR laser diode, at energy densities between 80 and 300 mJ/cm².Printing plate PP-12 was obtained.

3. ΔE Measurement

See example 1

4. Results

The total colour difference ΔE was measured and the results aresummarized in the Table 10 below.

A total colour difference ΔE above or equal to 2 is defined as a clearprint out image. A total colour difference ΔE below 2 is defined as anunsufficient print out image.

TABLE 10 ΔE of printing plate PP-12 Printing plate PP-12 ComparativeConcentration IR dye OC thickness Exposure energy mg/m² g/m² mJ/cm² ΔE0.157 2 g/m² 90 0.6 0.157 2 g/m² 120 0.4 0.157 2 g/m² 150 0.5

The comparative samples clearly demonstrate that the formation of aprintout image is suppressed when the total dry weight of the protectivetopcoat containing the thermochromic dye is too high, i.e. 2 g/m²: At asignificantly elevated dye concentration (157 mg/m²), the printout imageat exposure energies of 90 to 150 mJ/cm² remains very weak.

Example 3 1. Preparation of the Printing Plates PP-13 to PP-15Preparation of the Aluminium Support S-01

See Example 1.

Preparation of Inventive Printing Plate PP-13 and Comparative PrintingPlate PP-14 and PP-15. Photopolymerisable Layer

The photosensitive compositions as defined in Table 11 were coated ontothe above described support S-01. The components were dissolved in amixture of 35% by volume of MEK and 65% by volume of Dowanol PM(1-methoxy -2-propanol, commercially available from DOW CHEMICALCompany). The coating solution was applied at a wet coating thickness of30 μm and then dried at 120° C. for 1 minute in a circulation oven.

TABLE 11 composition of the photosensitive layer Ingredients g/m² PL-03PL-04 PL-05 FST 510 (1) 0.250 0.250 0.250 CN 104 (2) 0.250 0.250 0.250Initiator-01 (4) 0.045 0.045 0.045 S2539 (5) 0.020 0.020 0.020 Ruco coatEC4811 (6) 0.250 0.250 0.250 Tegoglide 410 (9) 0.0015 0.0015 0.0015Sipomer PAM 100 (10) 0.130 0.130 0.130 Albritect CP 30 (11) 0.024 0.0240.024 Black XV (12) — 0.020 — IR-01 (13) — — 0.020 1) FST 510 is areaction product from 1 mole of 2,2,4-trimethylhexamethylenediisocyanateand 2 moles of hydroxyethyl-methacrylate commercially available from AZElectronics as a 82 wt. % solution in MEK; 2) CN 104 is an epoxyacrylate oligomer commercially available from Arkema; 3) SR494 is anethoxylated pentaerythritol tetraacrylate commercially available fromArkema; 4) Initiator-01 is bis(4-tert-butylphenyl)-iodonium tetraphenylborate 5) S2539 is an infrared absorbing dye commercially available fromFEW Chemicals having the following structure:

6) Ruco coat EC4811 is a polyether polyurethane commercially availablefrom Rudolf Chemistry 7) S-LEC BX35-Z, S-LEC BL1 and S-LEC BL10 arepolyvinylbutyral resins commercially available from Sekisui; 8)Gohsefimer LL-02 is a polyvinyl alcohol having a low hydrolysis degreecommercially available from Nippon Goshei; 9) Tegoglide 410 is asurfactant commercially available from Evonik Tego Chemie GmbH; 10)Sipomer PAM 100 is a methacrylate phosphonic ester commerciallyavailable from Rhodia; 11) Albritect CP 30, is a copolymer ofvinylphosphonic acid and acrylic acid commercially available as a 20 wt.% aqueous dispersion from Rhodia. 12) Black XV is6-diethylamino-3-methyl-2-(2,4-xylidino) fluoran, a leuco dyecommercially available from Mitsui 13) IR-01 is an infrared absorbingdye having the following formula:

Protective Top Layer

On top of the photosensitive layer, a solution in water with thecompositions OC-13 and OC-14 as defined in Table 12 were coated (40 μm),and dried at 110° C. for 2 minutes. Printing plate precursors PPP-14 toPPP-16 having a coating thickness of respectively 0.54 and 0.49 g/m²were obtained.

TABLE 12 composition of the protective layers OC-13 and OC-14Ingredients OC-13 OC-14 g/m² inventive reference Mowiol 4-88 (1) 0.300.30 Mowiol 4-98 (1) 0.18 0.18 IR-01 (2) 0.05 — Lutensol A8 (3) 0.010.01 1) Mowiol 4-88 ™ and Mowiol 4-98 ™ are partially hydrolyzedpolyvinylalcohols commercially available from Kuraray; 2) IR-01 is aninfrared absorbing dye having the following formula:

3) Lutensol A8 ™ is a surface active agent commercially available fromBASF.

2. Imaging

The printing plate precursors PPP-13 to PPP-15 were imaged at 2400 dpiwith a High Power Creo 40W TE38 thermal platesetter™ (200 Ipi AgfaBalanced Screening (ABS)), commercially available from Kodak andequipped with a 830 nm IR laser diode, at energy densities between 50and 200 mJ/cm2. Printing plates PP-13 to PP-15 were obtained.

3. ΔE measurement

See example 1

4. Results Total Colour Difference ΔE

The total colour difference ΔE was measured and the results aresummarized in the Table 13 below.

A total colour difference ΔE above or equal to 2 is defined as a clearprint out image. A total colour difference ΔE below 2 is defined as anunsufficient print out image.

TABLE 13 Results of the total colour difference ΔE Printing PhotopolymerOvercoat Exposure energy Plate layer layer (mJ/cm2) ΔE PP-13 PL-03 OC-13120 5.3 inventive PP-14 PL-04 OC-14 120 1.7 comparative PP-15 PL-05OC-14 120 1.8 comparative

The results in Table 13 demonstrate that the print-out image is strong(the total colour difference ΔE is equal or higher than 2) for theinventive printing plate including an overcoat layer including the dyeaccording to the present invention. Comparative printing plates PP-14and PP-15 show a weak print-out image (the total colour difference ΔE islower than 2). Comparative printing plates PP-14 and PP-15 include no IRcompound according to the present invention in the overcoat layer.Printing plate PP-14 includes a leuco dye of the prior art in thephotosensitive layer and PP-15 includes the IR dye according to thepresent invention in the photosensitive layer. Both plates result in apoor print-out image.

5. Stability of the Print-Out Image

The stability of the print-out image was evaluated by determining thetotal colour difference ΔE before and after exposing a printing plate toregular white office light (800 lux). As a reference, the total colourdifference ΔE was also determined of a printing plate which was storedin a bag protected from external light.

The total colour difference ΔE of the printing plates was measuredimmediately after the exposure step (see below). Subsequently, theprinting plates were exposed to regular white office light (800 lux) aswell as stored in a bag protected from external light for one hour. TheΔE was re-measured after one hour and the obtained results aresummarized in Table 14 below.

The total colour difference ΔE was measured immediately after exposureto regular white office light (800 lux) and after a time period of onehour.

The total colour difference ΔE of the printing plates was measuredimmediately after the exposure step. Subsequently, the printing plateswere exposed to regular white office light (800 lux) as well as storedin a bag protected from external light for one hour. The ΔE wasre-measured after one hour and the obtained results are summarized inTable 14.

TABLE 14 Effect of light exposure to the print-out image Printing Totalcolour difference ΔE * Plate initial One hour white light One hour in abag PP-13 5.3 4.1 5.0 inventive PP-14 1.7 <2 <2 comparative * SeeExamples 1 and 2

The results clearly demonstrate the excellent stability of the print-outimage obtained for the inventive printing plate including the IR dyeaccording to the present invention in the protective overcoat layer. Thetotal colour difference ΔE remains high, even after exposure to officelight and/or storage in a bag for one hour.

The print-out image of the comparative printing plate PP-14 is weak andfades away after storage of one hour. The total colour difference ΔE islow and decreases further after storage in a bag and/or after exposureto regular white office light (800 lux).

1-15. (canceled)
 16. A lithographic printing plate precursor comprisinga support and a coating comprising (i) a photopolymerisable layercomprising a polymerisable compound, a borate compound, and aphotoinitiator, and (ii) a toplayer above the photopolymerisable layer;wherein the toplayer has a thickness between 0.1 g/m² and 1.75 g/m²,comprises an infrared absorbing compound comprising a thermocleavablegroup which, upon exposure to heat and/or IR radiation, transforms intoan electron-donor group which is a stronger electron-donor than thethermocleavable group, and is capable of forming a print-out image uponexposure to heat and/or IR radiation.
 17. The printing plate precursorof claim 16, wherein the coating is capable of being developed on-presswith dampening liquid and/or ink.
 18. The printing plate precursor ofclaim 16, wherein the borate compound is of the formula

wherein R_(b) ¹, R_(b) ², R_(b) ³ and R_(b) ⁴ are independently anoptionally substituted aliphatic hydrocarbon group, an optionallysubstituted aryl, or an optionally substituted heteroaryl group; and M⁺is an alkali metal cation or an optionally substituted onium ion. 19.The printing plate precursor of claim 18, wherein M⁺ is Li⁺, Na⁺, or K⁺.20. The printing plate precursor of claim 16, wherein the infraredabsorbing compound is an infrared absorbing dye which has a mainabsorption in the infrared wavelength range of the electromagneticspectrum before exposure to heat and/or IR radiation, and the infraredabsorbing dye absorbs more light in the visible wavelength range of theelectromagnetic spectrum after exposure to heat and/or IR radiation. 21.The printing plate precursor of claim 16, wherein the infrared absorbingdye is of Formula I, II or III:

wherein Ar¹, Ar² and Ar³ independently are an optionally substitutedaromatic hydrocarbon group or an aromatic hydrocarbon group comprisingan optionally substituted annulated benzene ring; W¹ and W²independently are a sulphur atom, an oxygen atom, NR″ wherein R″ is anoptionally substituted alkyl group, NH, a —CM¹⁰M¹¹ group wherein M andM¹¹ are independently an optionally substituted aliphatic hydrocarbongroup or an optionally substituted (hetero)aryl group, or M¹⁰ and M¹¹together comprise the necessary atoms to form a cyclic structure; W³ isa sulphur atom or a —C(A³)=C(A⁴)-group; W⁴ is a sulphur atom or a—C(A⁷)=C(A⁸)-group; M¹ and M² independently are hydrogen, an optionallysubstituted aliphatic hydrocarbon group, or together comprise thenecessary atoms to form an optionally substituted cyclic structure whichmay comprise an optionally substituted annulated benzene ring; M³ and M⁴independently are an optionally substituted aliphatic hydrocarbon group;M⁵, M⁶, M⁷, M⁸, M¹⁶ and M¹⁷ independently are hydrogen, a halogen, or anoptionally substituted aliphatic hydrocarbon group; A¹, A², A³, A⁴, A⁵,A⁶, A⁷, and A⁸ independently are hydrogen, a halogen atom, an optionallysubstituted aliphatic hydrocarbon group, an optionally substituted(hetero)aryl group, or A¹ and A², A³ and A⁴, A⁵ and A⁶, or A⁷ and A⁸together comprise the necessary atoms to form a cyclic structure; M¹²,M¹³, M¹⁴, and M¹⁵ independently are an optionally substituted aliphatichydrocarbon group; an optionally substituted (hetero)aryl group; two ofM¹⁴, M¹⁵, A⁵, and A⁷ together comprise the necessary atoms to form atleast one cyclic structure; or two of M¹², M¹³, A², and A⁴ togethercomprise the necessary atoms to form at least one cyclic structure; andM⁹ is a group that can be transformed by a chemical reaction, IRradiation, or heat into an electron-doner group that is a strongerelectron-doner than M⁹ such that an integrated light absorption of thedye between 350 to 750 nm increases; wherein the dye optionallycomprises one or more counter ions in order to obtain an electricallyneutral compound.
 22. The printing plate precursor of claim 21, whereinM¹ and M² together comprise the necessary atoms to form an optionallysubstituted 5- or 6-membered ring, which may comprise an optionallysubstituted annulated benzene ring.
 23. The printing plate precursor ofclaim 22, wherein the infrared absorbing dye is of Formula IV

wherein Ar¹, Ar², W¹, W², M¹, M², M³, M⁴, M⁵, M⁶, M⁷, M⁸, and M⁹ are asdefined in claim 21 and wherein the dye optionally comprises one or morecounter ions in order to obtain an electrically neutral compound. 24.The printing plate precursor of claim 23, wherein M⁹ is—(N=CR¹⁷)_(a)—NR⁵—CO—R⁴, —(N=CR¹⁷)_(b)—NR⁵—SO₂—R⁶,—(N=CR¹⁷)_(c)—NR¹¹—SO—R¹², —SO₂—NR¹⁵R¹⁶, or—S—CH₂—CR⁷(H)_(1-d)(R⁸)_(d)—NR⁹—COOR¹⁸; wherein a, b, c and dindependently are 0 or 1; R¹⁷ is hydrogen, an optionally substitutedaliphatic hydrocarbon group, an optionally substituted (hetero)arylgroup, or R¹⁷ and R⁵ or R¹⁷ and R¹¹ together comprise the necessaryatoms to form a cyclic structure; R⁴ is —OR¹⁰, —NR¹³R¹⁴, or —CF₃; R¹⁰ isan optionally substituted (hetero)aryl group or an optionally branchedaliphatic hydrocarbon group; R¹³ and R¹⁴ independently are hydrogen, anoptionally substituted aliphatic hydrocarbon group, an optionallysubstituted (hetero)aryl group, or R¹³ and R¹⁴ together comprise thenecessary atoms to form a cyclic structure; R⁶ is an optionallysubstituted aliphatic hydrocarbon group, an optionally substituted(hetero)aryl group, —OR¹⁰, —NR¹³R¹⁴ or —CF₃; R⁵ is hydrogen, anoptionally substituted aliphatic hydrocarbon group, SO_(3,) a —COOR¹⁸group, an optionally substituted (hetero)aryl group, or R⁵ together withat least one of R¹⁰, R¹³, and R¹⁴ comprise the necessary atoms to form acyclic structure; R¹¹, R¹⁵ and R¹⁶ independently are hydrogen, anoptionally substituted aliphatic hydrocarbon group, or an optionallysubstituted (hetero)aryl group, or R¹⁵ and R¹⁶ together comprise thenecessary atoms to form a cyclic structure; R¹² is an optionallysubstituted aliphatic hydrocarbon group or an optionally substituted(hetero)aryl group; R⁷ and R⁹ independently are hydrogen or anoptionally substituted aliphatic hydrocarbon group; R⁸ is —COO- or—COOR^(8′) wherein R^(8′) is hydrogen, an alkali metal cation, anammonium ion, or a mono-, di-, tri- or tetra-alkyl ammonium ion; and R¹⁸is an optionally substituted (hetero)aryl group or an alpha-branchedaliphatic hydrocarbon group.
 25. The printing plate precursor of claim21 wherein M⁹ is —NR⁵—CO—R⁴ or —NR⁵—SO₂—R⁶ wherein R⁴ is —OR¹⁰, whereinR¹⁰ is an optionally branched aliphatic hydrocarbon group; R⁵ ishydrogen, an optionally substituted aliphatic hydrocarbon group, or anoptionally substituted (hetero)aryl group; and R⁶ is an optionallysubstituted aliphatic hydrocarbon group or an optionally substituted(hetero)aryl group.
 26. A method for making a printing plate precursorcomprising the steps of: coating a support with: (i) aphotopolymerisable layer comprising a polymerisable compound, a boratecompound, and a photoinitiator, and (ii) a toplayer above thephotopolymerisable layer including an infrared absorbing dye which has amain absorption in the infrared wavelength range of the electromagneticspectrum before exposure to heat and/or IR radiation, and the infraredabsorbing dye absorbs more light in the visible wavelength range of theelectromagnetic spectrum after exposure to heat and/or IR radiation, anddrying the precursor.
 27. A method for making a printing plate includingthe steps of: image-wise exposing a lithographic printing plateprecursor to heat and/or IR radiation to form a lithographic imagecomprising image areas and non-image areas and to induce a colour changein the image areas; and developing the exposed precursor, wherein thelithographic printing plate precursor comprising a support and a coatingcomprising (i) a photopolymerisable layer comprising a polymerisablecompound, a borate compound, and a photoinitiator, and (ii) a toplayerabove the photopolymerisable layer; wherein the toplayer has a thicknessbetween 0.1 g/m² and 1.75 g/m², comprises an infrared absorbing compoundcomprising a thermocleavable group which, upon exposure to heat and/orIR radiation, transforms into an electron-donor group which is astronger electron-donor than the thermocleavable group, and is capableof forming a print-out image upon exposure to heat and/or IR radiation.28. The method of claim 27, wherein the precursor is developed bymounting the precursor on a plate cylinder of a lithographic printingpress and rotating the plate cylinder while feeding dampening liquidand/or ink to the precursor.
 29. The method of claim 27, wherein thecolour change is characterized by a CIE 1976 colour distance ΔE betweenthe image and non-image areas of at least
 2. 30. The method of claim 27,wherein the energy density of the IR radiation is between 80 mJ/m² and120 mJ/m².